1. Field of the Invention
This invention generally relates to integrated circuit (IC) and liquid crystal display (LCD) fabrication and, more particularly, to a system and method for cleaning IC or LCD substrate surfaces with ozone.
2. Description of the Related Art
An LCD panel is indispensable for notebook type personal computers (PCs) because it is light in weight and thin in profile, as compared to a conventional cathode ray tube (CRT) monitor. The price of LCD monitors continues to decrease, but LCDs continue to be more expensive than CRT monitors. Hence, further reductions in the LCD fabrication process must occur if the LCD is to ever completely replace the CRT. In order to reduce costs in LCD fabrication, new stripping methods using ozone have been developed. In some processes the requirement of an organic strip has been eliminated altogether with the use of ozone strip and clean methods. Organic stripper is expensive and requires special treatment of the surface to be cleaned. Further, organic stripper is toxic and requires expensive waste processes.
As noted in U.S. Pat. No. 5,464,480 (Matthews), in the fabrication of semiconductor wafers, several process steps require contacting the wafers with fluids. Examples of such process steps include etching, photoresist stripping, and prediffusion cleaning. Often the chemicals utilized in these steps are quite hazardous in that they may comprise strong acids, alkalis, or volatile solvents. The equipment conventionally used for contacting semiconductor wafers with process fluid consists of a series of tanks or sinks into which cassette loads of semiconductor wafers are dipped. Such conventional wet processing equipment poses several difficulties.
First, moving the wafers from tank to tank may result in contamination that is extremely detrimental to the microscopic circuits created in the fabrication process. Second, the hazardous chemicals and deionized water which are used have to be regularly replaced with new solutions, usually introduced to the tank by bottle pour, chemical distribution or from building facilities in the case of deionized water. The chemicals generally are manufactured by chemical companies and shipped to the semiconductor manufacturing plant. The chemical purity is thus limited by the quality of the water used by the chemical manufacturers, by the container used for shipping and storing the chemical and by the handling of the chemical.
Moreover, as chemicals age, they can become contaminated with impurities from the air and from the wafers. The treatment of the last batch of wafers prior to fluid rejuvenation may not be as effective as treatment of the first batch of wafers in a new solution. Non-uniform treatment is a major concern in semiconductor manufacturing.
Some of the fluid contact steps of semiconductor manufacture include removal of organic materials and impurities from the wafer surface. For example, in the manufacture of integrated circuits, it is customary to bake a photoresist coating onto a silicon wafer as part of the manufacturing process. This coating of photoresist or organic material must be removed after processing.
Generally, a wet photoresist strip process is performed by a solution of sulfuric acid spiked with an oxidizer of either hydrogen peroxide or ozone. However, there are many disadvantages to using a solution of sulfuric acid and an oxidizer to strip photoresist from wafers during semiconductor manufacture. First, the by-product of the resist strip reaction when hydrogen peroxide is used as the a oxidizer is water, which dilutes the concentration of the bath and thereby reduces its ability to strip photoresist. Second, this process operates at a high temperature, generally between 80 degrees C and 150 degrees C., typically above about 130 degrees C, which mandates the use of special heat resistant materials and components in order to house, circulate and filter the solution, as well as requires extra energy to conduct the cleaning process. Third, the solution is hazardous to handle and dispose of and expensive to manufacture, transport and store.
Moreover, due to the build-up of impurities both dissolved and undissolved in the process bath, the solution must be changed periodically. Typically, the interval for chemical change out is about every eight hours. Because the chemical adversely affects the drain plumbing, the solution must be cooled to less than about 90 degrees C prior to disposal. Thus, use of this photoresist stripping process requires either the use of additional tanks to contain the hot solution or the shut down of the process station during the chemical change period, reducing wafer throughput and increasing cost of ownership.
Finally, after use of a sulfuric acid solution for removal of photoresist, the wafers must be rinsed in hot deionized water since sulfate residues may crystallize on the wafer during processing causing process defects.
Another process often utilized for the removal of organic and metallic surface contaminants is the xe2x80x9cRCA cleanxe2x80x9d process which uses a first solution of ammonium hydroxide, hydrogen peroxide, and water and a second solution of hydrochloric acid, hydrogen peroxide, and water. The RCA cleaning solutions typically are mixed in separate tanks. The wafers are first subjected to cleaning by the ammonium hydroxide solution, then are moved to a rinse tank, then to a tank containing the hydrochloric acid cleaning solution, and then to a final rinse tank. This process, like the sulfuric acid process, has the disadvantage of using strong chemicals. Moreover, the wafers are exposed to air during the transfers from tank to tank, allowing for contamination. Finally, the use of peroxide may cause the wafers to suffer aluminum contamination from the deposition of aluminum in the high pH ammonium hydroxide solution which is not totally removed in the hydrochloric solution.
Various approaches have been taken for improving the processes and equipment used to treat semiconductor wafers with fluid. These attempts to improve on present processes generally involve either a change in equipment or a change in the process chemicals.
U.S. Pat. No. 5,082,518 (Molinaro), describes a different approach to improving the sulfuric acid and oxidizer process of cleaning semiconductor wafers. The system in this patent provides a gas distribution system that includes a sparger plate with diffusion holes for distributing gas throughout the bath in the tank. The Molinaro patent provides an apparatus that distributes ozone directly into the treatment tank containing sulfuric acid. Other approaches use ozone-injected ultrapure water to clean organic impurities from silicon wafers at room temperature. Other methods increase the concentration of ozone in the water to improve the etch rates.
New stripping methods using ozone gas or ozonated water can reduce or eliminate organic stripper, currently used in thin film transistor (TFT) LCD manufacturing. Ozone stripping and cleaning processes are relatively easy to implement and are environmentally friendly. One problem with the use of ozone, however, is in the oxidation of exposed surfaces. Ozone itself is a strong oxidizer, so metal and silicon surfaces that are cleaned with ozone, or even just exposed, are likely to become oxidized. This surface oxidized layer increases the contact resistance between electrodes and buslines, and causes poor LCD panel quality. The critical contact resistance, which causes poor image quality, is strongly dependent on several factors, such as the TFT LCD display size, resolution, and busline materials. Surface oxidized layers need not aggravate image quality in all circumstances. However, the removal of the surface oxidized layer is still important factor in the stabilization of panel quality and the improvement of production yields.
It would be advantageous if the further use of ozone to clean IC and LCD substrate surfaces could be advanced to reduce the use of organic strippers.
It would be advantageous if ozone could be used in cleaning processes where the oxidation of substrate surfaces is an issue.
It would be advantageous if substrate surfaces, oxidized as a result of ozone cleaning, could be easily cleaned of such an oxide.
The present invention is useful in preventing an electrical connection failure due to substrate surface oxidation, caused by an ozone stripping and cleaning process. The present invention removes the surface oxidized layer before layers are deposited upon the ozone-cleaned surface.
Accordingly, a method is provided for ozone cleaning in the fabrication of LCDs. The method comprises: forming a substrate with a surface material selected from the group including tantalum (Ta), titanium (Ti), tantalum nitride (TaN), and titanium nitride (TiN), aluminum (Al), Al compounds, tungsten (W), chrome (Cr), copper (Cu), silicon (Si), and Si compounds; patterning a layer of photoresist overlying the substrate surface; cleaning the substrate surface with ozone to remove the photoresist; in response to the ozone cleaning, oxidizing the substrate surface; dry etching the oxidized substrate surface; and, depositing a metal such as indium tin oxide (ITO), Ti, Ta, and Al overlying the substrate surface.
Dry etching the oxidized substrate surface includes exposing the oxidized substrate surface to a dry etchant for a period of approximately 30 seconds. Typically, an etching gas such as Cl2, BCl3, CF4, SF6, or NF3 is used to support the etching process. The oxidized substrate surface is dry etched to a depth in the range of 20 xc3x85 to 200 xc3x85 to remove the oxidized layer.
Additional details of the above-described ozone cleaning process, and an alternate method of removing the ozone-oxidized layer of a substrate surface are presented below.